FIG. 4 is a diagram illustrating a conventional optical fiber communication system. In this system, an optical signal is transmitted through an optical fiber having dispersive characteristics. In FIG. 4, a pulse pattern generator 5 generates an input electrical signal waveform 1. A semiconductor laser diode (hereinafter also referred to as LD) 6 converts an input electrical signal waveform I into an input optical signal waveform 2. An isolator 7 limits the transmission of the optical signal to only one direction. A main optical fiber 8 several tens to several hundreds of km in length having dispersive characteristics is provided for transmitting a communication signal. An optical attenuator 9 attenuates the optical signal. A photodiode (hereinafter may be also referred to as PD) 10 converts the output optical signal waveform 3 into an output electrical signal waveform 4. An amplifier 11 amplifies the output electrical signal waveform 4. A bit error rate measuring apparatus 12 measures the transmission error rate of the optical signal transmitted through the main optical fiber 8 in response to the output of the amplifier 11.
The input electrical signal waveform 1 having a rectangular waveform is generated by the pulse pattern generator 5 and converted into the input optical signal waveform 2 having a rectangular waveform by the semiconductor laser diode 6. The input optical signal waveform 2 is input to the main optical fiber 8 through the isolator 7. The optical signal transmitted through the main optical fiber 8 having an entire length of several tens to several hundreds of km, for example, 40 km, is output to the photodiode 10 through the optical attenuator 9. This output optical signal waveform 3 is dispersed due to the dispersive characteristics of the main optical fiber 8, resulting in a smoothly-sloping and wide signal waveform as shown in FIG. 4. This output optical signal is received by the photodiode 10 and converted into an electrical signal. The electrical signal is amplified by the amplifier 11, thereby becoming the output electrical signal waveform 4 that is smoothly sloped and broadened due to the response characteristic of PD 10, as shown in FIG. 4. 0n the basis of this waveform 4, a code error rate is measured by the code error rate measuring apparatus 12. In this way, optical communication is carried out.
FIG. 5(a) shows an oscillation spectrum of the semiconductor laser diode 6 driven at a transmission speed of 2.5 Gb/s. The spectrum has five peaks within a range of wavelengths, .DELTA..lambda.=5 nm, between 1.305 and 1.310 .mu.m. The semiconductor laser diode 6 is driven by a pulsed signal having a completely rectangular waveform, as shown in FIG. 5(b), and outputs a pulsed optical signal having an approximately rectangular waveform, although the optical signal which is an oscillation output of the laser diode has a little distortion, as shown in FIG. 5(c), due to relaxation oscillation of the laser diode 6. Here, this pulsed optical signal having a rectangular waveform is produced by respective modes of the five oscillation peaks shown in FIG. 5(a) and pulsed signals having a completely rectangular waveform are synthesized together. This pulsed optical signal is transmitted through the main optical fiber 8 having the dispersive characteristics shown in FIG. 5(d).
The dispersion is produced by different transmission times of the light traveling through the main optical fiber 8 depending on wavelength, which gives different transmission times relative to the transmission time of a signal having a reference wavelength, i.e., the delay time as a relative value. In other words, the dispersion indicates a delay in the transmission of the pulsed optical signal relative to a signal having a wavelength at which the dispersion is zero. Therefore, the dispersive characteristic of the main optical fiber 8 has a positive inclination as a function of wavelength. Suppose that the dispersion of the main optical fiber 8 is 0 ps/nm/km at a wavelength of 1.250 .mu.m and it is 17 ps/nm/km at the center wavelength for a bandwidth of 1.305 to 1.310 .mu.m. When the light having a bandwidth of .DELTA..lambda.=5 nm, shown in FIG. 5(a), is transmitted through the main optical fiber 8 of 40 km length, the maximum delay is EQU 17 (ps/nm/km).times.5 (nm).times.40 (km)=3400 ps.
This delay is inherent in the rectangular waveform of the pulsed optical signal at the output side, and the output optical signal waveform 3 at the receiver is a distorted waveform having a width larger by 3400 ps than the input optical signal waveform 2 at the transmitter.
The prior art optical fiber communication apparatus has the described construction in which the input optical signal waveform 2 having a rectangular waveform that is converted into an optical signal travels through the main optical fiber 8 over an entire length of several tens to several hundreds of km. The output optical signal waveform 3 is not the same as the input optical signal waveform 2 due to the dispersion of the main optical fiber 8, Instead, the output waveform is smoothly sloping, thereby increasing the bit error rate.